In machine tools equipped with automatic tool change mechanisms it is usually necessary to stop the rotation of the spindle at a particular angular orientation, referred to as the keylock position. This is done to obtain meshing engagement of a key on the cutting tool with a keyway on the spindle when the tool is transferred into the spindle by the tool change mechanism. Such angular orientation of the spindle is achieved by means of a transducer such as a resolver or the like.
The output shaft of a motor for driving the spindle is coupled to the spindle through a gear train because of the wide range of speeds required for the spindle rotation. Even though the motor is controlled to be driven at different speeds, in order to obtain a wider range of speeds, the gear train is provided with a shiftable gear cluster that shifts into either a low range of speeds or a high range of speeds. As a result, during the shifting of the gears, the drive train from the motor to the spindle is momentarily interrupted.
It has been the practice in the past to drive the transducer directly from the spindle so that the transducer and spindle rotate in unison. With this arrangement the required synchronization between the spindle and the transducer to obtain accurate angular orientation of the spindle upon termination of its rotation presents no problem.
However, the difficulty with such an arrangement is that in order to obtain the desired angular orientation of the spindle, the motor shaft must be accurately stopped to obtain the positioning of the spindle. The motor is coupled to the spindle, and therefore to the transducer, through the gear train and an appreciable amount of backlash exists in the gear train. As a result, inaccuracies may be introduced between the motor and the transducer and hunting of the motor may occur.
It has been generally recognized that for precise control of the motor the ideal arrangement is for the transducer to be driven directly from the motor shaft. In one arrangement, the motor shaft drives a resolver which increments a register such that the count in the register at any given time precisely indicates the angular orientation of the motor shaft at the given time. Since the spindle rotates synchronously with the shaft through the gear train, the count also indicates the angular orientation of the spindle at any time, if it is known that a particular count represents a particular spindle orientation. However, during shifting of gears in the gear train coupling the motor to the spindle, the spindle may not rotate synchronously with the shaft and resolver, allowing the angular position of the spindle relative to the shaft and resolver to change by an unknown amount. Thus, the relationship between spindle orientation and the count contained in the register is no longer known.
This problem was dealt with in U.S. Pat. No. 4,449,866 which issued on May 22, 1984 to Lohneis et al. Such patent teaches an arrangement wherein a shiftable gear cannot move out of mesh with an engaged gear until it engages the teeth of a new gear. Thus, none of the gears are ever free to rotate without being in mesh with a complementary gear. With this system, the transducer can be driven directly off of the motor shaft and yet will not lose synchronism with the spindle even though the motor drives the spindle through a gear train with shiftable gears. However, a shifting action cannot take place unless all the gears are completely at rest.
The present invention is an arrangement wherein a motor shaft directly drives a resolver or other transducer device to generate an output representing the angular position of the shaft, and at the same time drives a spindle through a gear train. After the gears have been shifted, it is necessary to determine the angular position or orientation of the spindle with respect to the motor shaft so that the motor can be operated to stop the spindle in its keylock position. This could be done by means of structure which causes a signal to be generated when a reference point on the spindle passes a fixed reference point located proximate to the spindle. Spindle position is thus known when the signal occurs, and the transducer output at such time indicates the angular orientation of the shaft. Such information may be used to readily determine spindle position with respect to the motor shaft. Unfortunately, however, structure which is practical for use in such an arrangement may introduce the following errors: (1) Signals may be generated when the point on the spindle is anywhere within a small range of positions with respect to the fixed point, rather than at one specific point; and (2) the transducer device may not register counts quickly enough to represent the actual position of the motor shaft when a signal is generated.
In the invention, it has been recognized that the spindle can have only certain discrete angular positions with respect to the shaft when the spindle and motor shaft are engaged through the gear train. The number and specific values of these positions is determined by the gear ratio of the gear train, which includes a driving gear fixed for rotation with the motor shaft and a gear fixed to the spindle. The discrete spindle positions, in turn, correlate with discrete or specific values of transducer output. By comparing output values indicated by the transducer when one or more signals occur with the limited number of possible correlating transducer values, the angular position of the spindle with respect to the shaft can be accurately determined.